1. Field of the Invention
The present invention relates to medium telephoto lens systems and, more particularly, to a medium telephoto lens system having a large aperture ratio (hereinafter referred to as "A large aperture medium telephoto lens system").
2. Description of the Related Art
As medium telephoto lenses, Gauss type lenses and modified Gauss type lenses have heretofore been widely used because these lenses have excellent image-forming performance despite a simple lens arrangement composed of 6 to 8 lens elements.
Further, in recent years, so called "floating technique" has been introduced into medium telephoto lens systems of the type described above. Consequently, it has become possible to obtain excellent performance for objects in the object distance range of from the infinite object point to the nearest object point as far as flat image-forming performance is concerned.
Meanwhile, autofocus single-lens reflex cameras have recently spread remarkably. As a result, interchangeable lenses have also been demanded to reduce the weight of a lens unit movable during focusing and to shorten the distance through which the lens unit moves to effect focusing.
However, the focusing method used with the conventional Gauss type lens is to move the entire lens system to effect focusing or to vary a specific lens spacing while moving the entire lens system. In either case, the entire lens system, which is heavy, must be moved through a relatively long distance. Therefore, the conventional Gauss type lens is not suitable for use in an autofocus camera from the viewpoint of focusing speed and power consumption.
To solve the problem, the present applicant has proposed lens systems which are disclosed, for example, in Japanese Patent Application Laid Open No. 03-200909 and U.S. Pat. No. 4,812,027.
The lens systems disclosed in the above-mentioned publications are of the rear focusing type, which is composed of a front lens unit having three lens elements, i.e., a positive lens element, a positive lens element, and a negative lens element, based on the front lens group of the Gauss type lens, and a rear lens unit including a plurality of lens elements and having a positive refractive power as a whole. In the rear focusing type, focusing is effected by moving only the rear lens unit.
However, the rear focusing system suffers from the disadvantage that the closest distance performance (i.e., the image-forming performance for an object at the closest focusing distance) generally degrades in the case of "fast" lenses having an f-number smaller than 1.7, although it can make excellent aberration correction for relatively "slow" lenses having an f-number larger than 1.7.
The rear focusing system further involves the problem that the balance between spherical aberration and field curvature deteriorates at a specific object distance.
In view of the above-described problems, an object of the present invention is to provide a large aperture medium telephoto lens system which is capable of focusing by movement of a lens unit having a minimal number of lens elements and yet excellent in the closest distance performance.